It's two clusters, one centered around big data from 2000-2004, and another on large-scale machine-learning from 2011-2015. The middle period from 2004-2011 mostly just featured Spanner and helping other people pick up the torch for his accomplishments of 2000-2004. While Spanner is great and last I heard had one big client inside Google, it's hardly as impactful as GFS, Protobufs, MapReduce, Google's serving system, and BigTable were, nor as Brain, TensorFlow, and that other internal project are now.